1. Field of the Invention
The present invention relates to a dome-shaped diaphragm for loudspeakers and to a process for producing the diaphragm.
2. Description of the Prior Art
Conventionally dome-shaped diaphragms for loud speakers are produced chiefly by the following two methods in order to give the substrate of the diaphragm shape retentivity and airtightness.
When a substrate fabric of synthetic fiber is used for the first method, the substrate fabric is shaped to the form of a dome with application of heat to cause the fabric to retain the shape by utilizing the thermoplastic properties of the synthetic fiber itself. To assure higher shape retentivity, it is also practice to immerse the substrate fabric in a solution of phenolic resin and thereby utilize the thermosetting properties of the resin. Alternatively when a substrate fabric of natural fiber is used, it is critical to utilize the setting properties of such phenolic resin in affording the shape retentivity. The substrate fabric is rendered airtight, for example, by coating the substrate fabric with an emulsion of acrylate.
Since the first method forms a coating of resin emulsion to obtain airtightness, the method involves difficulty in giving a uniform thickness to the diaphragm, consequently failing to impart uniform elasticity to the diaphragm and permitting marked partial resonance in the range of pertial vibration. Further if the substrate fabric of natural fiber has large meshes or openings, the resin solution is likely to ooze from the rear side of the fabric, so that there is the need to use a thick substrate fabric of compact structure. This entails the drawback of adding to the weight of the diaphragm and resulting in a reduced acoustic radiation efficiency. Additionally it is impossible to coat the substrate fabric to a thickness larger than its thickness, and there is a limitation on the thickness of the coating.
Another method has been provided to overcome such drawbacks of the coating method, as disclosed, for example, in U.S. Pat. No. 4,140,203, wherein a polyurethane elastomer film is joined to a substrate fabric of natural or synthetic fiber with application of heat and pressure. Since the film to be joined has a uniform thickness, the second method is free of the foregoing drawbacks of the coating method.
To be sure, the second method joins the substrate fabric and the film together with heat and pressure, but the assembly, even if obtained under an increased pressure, is apparently composed of two layers and therefore has the drawback that when it is vibrated, the assembly is unable to permit continuous, smooth and proper propagation of the vibration across the junction of the substrate fabric and the film.
FIG. 4 shows the acoustic characteristics determined by an experiment for a loudspeaker diaphragm prepared by the second method described above. In this drawing, A represents sound pressure and B second harmonics. The second harmonics B exhibit 70 dB at 900 Hz, indicating pronounced distortion of sound due to partial vibration.
We conducted various experiments and found that this drawback of the prior art was attributable to the fact that the substrate fabric and the polyurethane elastomer film were directly joined together with heat and pressure to form an assembly of two layers.
Accordingly, instead of thermally joining the substrate fabric and the polyurethane elastomer film together under pressure, we attempted to adhere the fabric and the film together with an adhesive, for example, with epoxy resin.
FIG. 5 shows the acoustic characteristics determined by an experiment for a loudspeaker diaphragm which was prepared from a substrate fabric of synthetic fiber and a polyurethane elastomer film by joining them together with the adhesive.
The drawing shows that although sound pressure A' almost resembles that shown in FIG. 4, second harmonics B' exhibit 59 dB at 900 Hz, thus revealing slightly reduced distortion of sound.
However, a great improvement still remained to be made for actual use.